As the global market for electric vehicles grows over the next several years, lithium (Li) batteries are emerging as the technology of choice for auto manufacturers. Energy storage units for EV applications require high charge and discharge rates, long lifetimes, high energy density, and above all else, safe operation under normal and adverse conditions. Currently, the practical energy density of current, state-of-the-art Li-ion batteries of only 200-300 Wh/kg, which is more than one order of magnitude lower than the energy density of coal, 6.7 kWh/kg, and approaching two orders of magnitude lower than commercial grade gasoline, ˜12 kWh/kg. Even after factoring in the relatively low efficiency, ˜18%, of the internal combustion (IC) engine, relative to an electric motor (typically >60%), it is apparent that fossil fuels provide substantially greater energy storage capacity than current batteries. This means that next generation Li-ion batteries capable of achieving operating energy densities in the transportation sector that are competitive with conventional fossil fuels will require substantial improvements in all components of current state-of-the art Li-ion technologies.
The electrolyte in a battery functions to shuttle ions between the electrodes. This component of lithium ion batteries has historically received less attention than the electrodes, because the demands on voltage, longevity, power, and safety in portable device applications are modest in comparison to applications in transportation, where much larger battery packs are demanded. Current state of the art liquid electrolytes in Li-ion batteries targeted for EVs and HEVs are in most cases simple transplants from Li-ion batteries used in low-voltage, low-power, portable devices. Optimized largely for the rate at which they transport ions, the volatility, flammability, and instability of currently used carbonate electrolytes have largely been overlooked. These efforts have instead been given to development of additives that react selectively to form conditioning films that limit reactivity of the electrolyte with the electrodes, and flame-retarding additives and containers which limit the severity and scale of fires produced when combustible electrolytes or components ignite.
Recent progress in synthesis and electrochemical analysis of room temperature ionic liquids (ILs) has established the promise of this unique class of materials as electrolytes for next-generation lithium batteries. Ionic liquids are organic salts having melting points below 100° C. and generally consist of a bulky cation and an inorganic anion. The large cation size allows for delocalization and screening of charges, resulting in a reduction in the lattice energy and thereby the melting point or glass transition temperature. Ionic liquids have negligible vapor pressure, non-flammability, good room-temperature ionic conductivity, wide electrochemical window, and favorable chemical and thermal stability, which make them an attractive option as an electrolyte for lithium batteries.
Despite the obvious promise, ionic liquid electrolytes suffer from two physical property limitations that to-date has made them unattractive for lithium battery applications. First, the fraction of the ionic conductivity of the electrolyte arising from mobile lithium ions (i.e. the so-called lithium transference numbers) are typically low for these materials, which makes cells assembled using IL electrolytes prone to polarization. Second, most ionic liquids exhibit low tensile and compressive strengths, which allows them to leak and spill, which limits the form factors an IL-based lithium battery can take.